The functional and anatomical arrangement of a homogeneous population of neurons in the inferior colliculus (phasic constant latency responders (pCLRs) of Mexican freetailed bats and their importance for evaluating target distance during echolocation are being investigated. These neurons are distinguished by a phasic-on firing pattern evoked by brief downward sweeping frequency modulated (FM) pulses that stimulate the natural orientation cries and by highly constant firing latencies to repetitive signal presentations. Moreover, preliminary evidence strongly suggests that each pCLR is triggered by only one frequency component of the FM pulse and thus is responding to instantaneous frequency. Preliminary evidence also indicates that the pCLRs are stacked vertically within the colliculus and thus exist in vertical arrays. Assuming the pCLRs are tonotopically organized, each array of pCLRs should encode a biosonar signal in both the frequency and time domains. The frequency (spectrum) should be encoded by a place mechanism while the temporal position of each frequency component should be encoded in the firing latency as viewed across a vertical array of pCLRs. This project is designed to further investigate the features of the pCLRs and to elucidate the anatomical locus and arrangement of this neuronal population in the colliculus in order to better understand how, as a population, the pCLRs can provide the substrate for the matched filters shown to be used by bats in behavioral studies.